1. Field of the Invention
The present invention relates to heaters in an acid and rinse bath, and more specifically, to a method and an apparatus for detecting the crack of a heater in an acid and rinse bath.
2. Description of the Prior Art
There are hundreds of steps in integrated circuit manufacturing processes to the semiconductor devices on a wafer, for example, thermal oxidation process, photolithography process, etching process or cleaning process. Among others, chemical cleaning process is used very frequently in a semiconductor factory for wafer cleaning of particle, organic, and metal contamination. In general, the chemical cleaning processes use a series of cleaning steps. The chemical cleaning processes are designed for different kinds of semiconductor device pollution, especially for cleaning some contaminations such as metal silicide, refractory metal layer, aluminum, sputtered glass layer and deposited glass layer. Chemical cleaning processes for Si layer or SiO.sub.2 layer are performed before proceeding with any high temperature processes such as an oxidation process, diffusion process, deposition process or annealing process.
There are several chemical cleaning processes in normal, such as stripping the residue photoresist on substrate by using plasma, then being submerged into H.sub.2 SO.sub.4 -H.sub.2 O.sub.2 solution to clean the surface of the substrate and maintain the temperature between 18 to 23 degrees centigrade; likely, it is also possible for cleaning the surface of the substrate by deionized water(DI water), then use heated N.sub.2 gas to dry. Above mentioned cleaning processes are often used in wafer fabrication.
Conventional chemical cleaning is performed with a series of acid and rinse baths. A mixture of NH.sub.4 OH, H.sub.2 O.sub.2 and H.sub.2 O is prepared in acid and rinse bath and heated to 60-80.degree. C. The wafers are submerged in the solution for 10-15 minutes, then removed and rinsed in DI water. The usage of wafer cleaning with an acid and rinse bath can remove organism or metal contaminations on the surface of the substrate.
The temperature of the solution in the acid and rinse bath should be maintained around but not above 80.degree. C. A heated filament is submerged in the solution in order to maintain the temperature of the solution. Normally the heated filament is inserted into a quartz tube in order to avoid the corrosive solution reaction with the filament and cause severe pollution of the wafers.
Referring to FIG. 1, a cross sectional view of an acid and rinse bath 1000 is shown. Acid solution 1100 mixed by (NH.sub.4 OH): H.sub.2 O.sub.2 : H.sub.2 O in about (0.1.about.1): 1:5 by volume is filled into the acid and rinse bath 1000. The mix ratio of the solution depends on the different processes.
Still with reference to FIG. 1, heater 2000 is submerged into acid solution 1100 to heat the solution. The exterior shell of the heater 2000 is a hollow quartz tube 2100 with one open end and one closed end. Part of the quartz tube 2100 is submerged in acid solution 1100 with closed end thereof, another end of the quartz tube 2100 stretching out of the acid solution 1100. Heated filament 2200 is inserted into quartz tube 2100 from the open end of the quartz tube 2100 to reach to the closed end of the quartz tube 2100. The acid solution 1100 is heated by the electrically connected heated filament 2200 via quartz tube 2100.
Prolonged heating times are necessary since acid solution 1100 should always be maintained between 75 to 80.degree. C. Since the quartz tube 2100 will be corroded by acid solution 1100, long term immersion will cause heated filament 2200 to pollute the acid solution 1100; silicon wafers will also get contaminated and must be scrapped.
Unfortunately, a crack in the quartz tube 2100 is buried in acid solution 1100 and not easy to find out during operation. Therefore, it is desirable to have a detecting device to detect a crack in the heater of an acid and rinse bath. This crack detection device shall inform the operator to change the defective parts in order to maintain good wafer yields and reduce wafers loss.